Recovery of silver from residues containing the same



Patented May 31, 1949 RECOVERY OF SILVER FRGM RESIDUES CUNTAINING THESAME Herman A. Liebhafsky and Lester B. Bronk, Schenectady, N. EL,assignors to General Electric Company, a corporation of New York NoDrawing. Application August '7, 1946, Serial No. 688,968

7 Claims.

The present invention relates to a process for the recovery of silver orsilver compound from a contact mass employed in the preparation. oforgano-substituted halogenosilanes and is con cerned with a process forrecovering s" ent in the free or combined state) t-.. silver nitratefrom a contact mass can (a) silicon and (b) a composition or mate lectedfrom the class consisting of metallic sil silver halides, and mixturesof silver and silver halides over which mass there has been passed ahydrocarbon halide, (e. g., an alkyl halide, such as, for instance,methyl chloride, ethy" l ethyl chloride, etc; an aryl ha ide, e. g.,:hlorobenzene, bromobenzene, etc.; an ara yl halide, e. g., benzylchloride, etc; an alkaryl halide, e, g., tolyl chloride, etc.) at anelevated temperature, 6. g., above 250 C. More particularly, the processof the instant invention comprises firing the residue resulting from thepassage at an elevated temperature of the hydrocarbon halide over thecontact mass, the firing being effected in a nonoxidizing atmosphere ata temperature above 6&0" C., and thereafter treating the fired residuewith strong nitric acid (e. g., an aqueous nitric acid solution having aspecific gravity of from about 1.1 to 1.5, preferably from about 1.3 to1.415) and recovering the silver nitrate thus formed. The term silver,as used throughout the specification and claims in conjunction with theafore mentioned residue, is intended to refer to silver, either in thefree state, e. g., metallic silver, or in the combined state, e. g., asthe silver halide, etc.

Metallic silver and silver chloride (or mixtures of the two) are capableof accelerating the reaction between a hydrocarbon halide and silicon atelevated temperatures to yield hydrocarbonsubstituted halogenosilanes.In this connection it is believed that when a silver halide is crop inthe contact mass, reducing elements pres, the reaction, e. g., thesilicon, will reduce silver halide to metallic silver. The t. tallicsilver as a catalyst for the above-6 preparation of contact masses compH silicon and silver, is more fully disclo ed and claimed in PatnodePatent U. S. 2,380,997, issued August 7, 1945, and as i, ed to the assignee as the present invention. Metallic silver as a catalyst isespecially desirable in the of the preparation of aryl halogenosilanes,e. g., phenyl chlorosilanes resulting from the reaction of chlorobenzeneand the silicon in, for example, the silver-silicon contact mass. Theyields of aryl halogenosilanes obtained by using silver as a catalystare usually better than those obtained by using other metalliccatalysts. Since metallic silver nitrate.

silver and silver halides are quite expensive, it is highly desirablethat the silver and silver halides employed in the above-describedreaction be recovered as economically and as easily as possible so thatthey may be used in later synthesis of organo-substitutedhalogenosilanes.

Many attempts have been made to recover com.- pletely the silver(whether in the form or" free silver or in a combined form, e. g., as asilver halide) from the residue resulting from the passage of ahydrocarbon halide over the aforementioned contact mass. However,heretofore, the usual methods employed for recovering silver from suchresidues have resulted in unsatisfactory recoveries of the silver fromthe residue. For example, one of the approved methods for recoveringmetallic silver from masses containing the same comprises treating thesilvercontaining mass with nitric acid to convert the silver to However,when the residue obtained after passage of the hydrocarbon halide overthe contact mass comprising, for instance, silicon and silver is treatedwith strong nitric acid, the recovery of the silver (as silver nitrate)contained in the said residue is very small, rarely exceeding more thanabout one-hali oi the amount of silver contained therein.

Assuming that the silver in the residue is pres ent in the form of asilver halide, for example, as silver chloride, it would be expectedthat the usual chemical methods for recovering the silver halide wouldextract the silver from the residue. Thus, when the aforementionedresidue (after passage of the hydrocarbon halide thereover) treated withpotassium cyanide, there is a substantially incomplete recovery of anysilver chloride present in the residue, the amount recovered rarelyexceeding one-quarter of that present in the residue. It is, therefore,apparent that two of the usual methods for recovering silver from theabove-described residue are entirely inadequate for the purpose, andthat considerable loss would occur if only partial recovery of thesilver present, for instance, as free silver or as a silver halide,could be had from the residue.

We have now discovered that the silver contained in a residue resultingfrom the pass "e of a hydrocarbon halide at elevated temper ires over acontact mass comprising silicon a inaterial selected from the classconsisting of silver, a silver halide and mixtures of silver and a silvr halide, may be recovered in an almost quant ative yield, in the formof silver nitrate, if the aforementioned residue is first fired at atemperature above 600 C. in an atmosphere free of ii dizing agents.According to our invention, when the residue is thus fired, the silvercan be extracted from the residue almost quantitatively in the form ofsilver nitrate by thereafter treating the said fired mass with strongnitric acid and,

3 d if desired, isolating the silver nitrate fro passed was removed andsubjected to the fol mixture-"comprising nitric acid and the lowingtest: due. By our method, any silver in the (a) Ten parts of the aboveresidue were fired whether in the free or combined state ca for sixhours a hydrogen atmosphere at 600 be dissolved in strong nitric acid byim C. 'ili ei"eafter, the silver contained in the fired the firedresidue in the nitric acid resi us was extracted twice with concentratedably, though not essentially, heating the nitric a id. When the twoextracts were com- The manner whereby the above-described rebined andtitrated with standard ammonium sults are obtained is not clearlyunderstood. It thiccyanate solution using ferric alum as the inwasoriginally believed that the elevated tern" dicator, it was calculatedthat substantially all peratures (e. g., 250 to 600 C.) at which the reof the silver (present either in the uncombined action between thehydrocarbon halide and the form, 6. g., as metallic silver, or in thecombined silver-silicon contact mass (or silver halide form, e. g,silver chloride) was recovered from silicon contact mass) was effected,caused a the residue. change in the chemical or physical structure of(23) Another sample of the above-obtained the silver or silver halidepresent therein. To residue was digested in concentrated nitric acidtest this possibility, metallic silver was freshly refluxing the samefor about 12 hours prepared by precipitating the same and conduct-(omitting the firing step employed in (a) above).

with concentrated nitric acid to extract the silver ing-the followingtest: About 90 parts powdered lutration and titration of the filtrate bythe silicon and 10 parts of the freshly precipitated thiocyanate methodfailedto dissilver were mixed together to form a contact mass closeeven. a trace of silver nitrate. Treatment which was fired in a hydrogenatmosphere at 986 of a sample of the above-obtained residue C. for 45minutes; thereafter the mass was broken ittin th h at-tr tme t d t i idinto small particles. Substantially all the silver treatment) with about10 parts ofa 10 per cent contained in the fired mass was recovered as 35aqueous potassium cyanide solution on a steam silver nitrate when thefired mass was treated bath, resulted in the extraction of onlyapproxione-fiith of the silver (in whatever state ed) contained in theresidue.

in the form of silver nitrate. This indicated that the temperatureconditions at which the firing Note-In the above tests and the testswhich f the mixture of powders occurred were not the follow, theextraction of the residues was concause of converting the metallicsilver to a rela- -t W5 fgflov part ofthe powdered tivelynitric-acid-insoluble form. idue about 21 parts concentrated nitric acidAssuming that the silver contained in the concific gravity 1.4L) wereheated on a hot plate tact mass was converted to silver chloride by theC. for about it) minutes. The mixture passage of the hydrocarbonchloride during the was then centrifuged, and the residue Washedreaction, it was (160101601 to determine Wi'lGllllCl with gtgr the acidand washings being 0111.. the elevated temperatures caused the silverchlobined. Other successive extractions were carried ride to convert toa form which rendered it difh t imil l cult to extract the silverchloride by an approved method, namely, by the cyanide method. Example 2For this purpose, freshly precipitated silver chloresidue obtained inExample 1 as aresult ridewas dividedinto two portions. One portion ofthe passage oi chlorobenzene over the silverwas fused at a temperatureof about 450 C. (the silicon contact mass was fired at about 600 .0.temperature at which the reaction between the for hp oxi a ly 15 hoursin a nitrogen atmoshydrocarbon chloride and the silicon in the phere.Substantially all the silver contained .in silver-silicon contact massoften is caused to take the residue was recovered by the nitric acidexplace) while the other portion remained unfused. traction methodemployed in part (a) of Each portion was then immersed-in a 10 per centample 1. aqueouspotassium cyanide solution. Both por- Example 3 tionsdissolved readily in the solution indicating that the elevatedtemperatures encountered in Firing in 0f the residue resulting from thet aforementioned reaction were t t sole passage of chlorobenzene overthe silver-silicon cause of t Complication whereby neither nitric mass(see Example 1) resulted in the recovery acid nor potassium cyanide,which are approved of P Q Y y One-quarter 0f t e Silver methods forrecovering silverand silver chloride, 111E551? m the Saldresiduerespectively, could extract the silver or silver It is to beunderstood that Otherinert t OS- ch1oride pheres and firing temperaturesmay be-employed In-orcler that those skilled in the art may fully inaddition to those fifid in the foregoing examunderstand the manner inthis inve tion Illes- T111113, WE Page temperatures as has may bpracticed th fonowing exampges are 1500 C. for firing the residue inthenon-oxid-izing given by Way of illu tr tion and not by Way fatmosphere. However, we have found that itis 1i it ti l parts are bWeight seldom necessary to fire the residue in. the atmosphere free ofoxidizing agents at a temperature Example 1 85 much above 100G C. Thetime forfiring these About 10 parts silver powder and 90 partssilresidues, although it is not critical, may also .be icon powder werethoroughly mixed together and varied, e. g., from about 3 to 2% hours,preferably fired at 900C. for l5 minutes in a hydrogen at from about 6to 18 hours, depending e. g, on the mosphere. The mass was thereafterbroken into composition of the residue, temperature at-which smallpieces and packed into a reaction tube. the firing is conducted, etc.Chlorobenzene was passed through the tube for Persons skilled in the artwill also understand about 408 hours at a temperature of from about thatthis invention is applicableto, other residues 375 to 520 C. to yield areaction product comobtained in the same manner as in (Example "1prisinga mixture of phenyl chlorosilanes. The wherein the proportion byweightof silver-or residue in the tube comprising the silver-siliconsilver halide (or mixtures of thetwo) ,presentin contact mass over WhlChthe chlorobenzene had the original contact mass is greater or less thanthat employed in the above examples. For example, the amount of silverin the silver-silicon mass, prior to passage of the hydrocarbon halidemay comprise from as little as 2 per cent to as much as 50 per cent ormore by weight of the total mass of silver and silicon.

One of the practical applications of our invention comprises recoveringthe silver in the form of silver nitrate from the residue resulting from,the passage of a hydrocarbon halide over a contact mass comprising, forexample, silicon and silver, and thereafter treating the said silvernitrate, either in the recovered or isolated state or in the form of thenitric acid solution from which substantially all the solid insolublematter has been removed, with hydrogen chloride or hydrochloric acid toconvert the silver nitrate to silver chloride. The silver chloride thusobtained may then be employed as a catalyst for efi'ecting reactionbetween a hydrocarbon halide and the silicon component of a contact masscomprising silicon and the said silver chloride.

What we consider new and desire to secure by Letters Patent of theUnited States is:

1. The process which comprises (1) firing a residue obtained by passageof a hydrocarbon halide over a contact mass comprising (a) silicon and(b) a material selected from the class consisting of silver, silverhalides, and mixtures of silver and silver halides at an elevatedtemperature, said firing taking place at a temperature above 600 C. inan atmosphere free of oxidizing agents, (2) thereafter treating thefired residue with strong nitric acid, and (3) recovering the formedsilver nitrate.

2. The process which comprises (1) firing a residue obtained by passageof a hydrocarbon halide over a contact mass comprising (a) silicon and(1:) silver at an elevated temperature, said firing taking place at atemperature above 600 C. in a non-oxidizing atmosphere, (2) thereaftertreating the fired residue with concentrated nitric acid, and (3)recovering the formed silver nitrate.

3. The process which comprises (1) firing a residue obtained by passageof a hydrocarbon halide over a contact mass comprising (a) silicon and(b) a silver halide at an elevated tempera ture, said firing takingplace at a temperature above 600 C. in a non-oxidizing atmosphere, (2)thereafter treating the fired residue with concentrated nitric acid, and(3) recovering the formed silver as nitrate.

4. The process which comprises (1) firing a residue obtained by passageof a halogenobenzene nti) over a contact mass comprising silver andsilicon at a temperature above 250 C., said firing taking place at atemperature above 600 C. in a nonoxidizing atmosphere, and (2)thereafter treatin the fired residue with concentrated nitric acid and(3) isolating the silver nitrate thereby formed.

5. The process which comprises (1) firing a residue obtained by passageof chlorobenzene over contact mass comprising silver and silicon at atemperature above 250 0., said firing taking place at a temperatureabove 600 C. in a hydrogen atmosphere, (2) thereafter treating the firedresidue with concentrated nitric acid and (3) isolatthe silver nitratethereby formed.

6. The process which comprises (1) firing a residue obtained by passageor" chlorobenzene over a contact mass comprising silver and silicon at atemperature above 250 C., said firing taking place temperature above 600C. in a nitrogen atmosphere, (2) thereafter treating the fired residuewith concentrated nitric acid and (3) isolatthe silver nitrate therebyformed.

'7. The process which comprises (1) firing a residue obtained by passageof chlorobenzene over a contact mass comprising silver and silicon at atemperature above 250 C., said firing taking place at a temperatureabove 600 C. in an atmosphere free of oxidizing agents, (2) thereaftertreating the fired residue with concentrated nitric acid, (3) removinsubstantially all the undissolved portions from the nitric acidsolution, (4) adding hydrochloric acid to the liquid nitric acidsolution substantially free of solid matter to precipitate silverchloride, and (5) recovering the precipitated silver chloride.

HERMAN A. LIEBHAFSKY. LESTER B. BRONK.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,380,996 Rochow et a1. Aug. 7,1945 2,380,997 Patnode Aug. 7, 1945 2,380,998 Sprung et al Aug. '7, 19452,380,999 Sprung et al. Aug. '7, 1945 OTHER REFERENCES Mellors,Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 3,Longmans, Green 8: 00., N. Y., publishers, pages 392, 398 and 459.

